Natural gas from the wellhead is a mixture of various different gases including methane, ethane, propane, butane, etc. Natural gas liquid ("NGL") processing plants liquefy and extract the ethane, propane, butane, etc. and sell these products as feedstock to petrochemical plants and refineries or to distributors to be sold as a home heating fuel. All of the ethane and much of the propane from NGL plants ultimately are used as feedstock in ethylene plants. Here the ethane and propane are cracked into ethylene and propylene which are themselves then used as feedstock in various chemical and plastic processes. Today, the NGL plant and the ethylene plant are totally separated with the only connection between them being a pipeline. The present invention brings components of these two plants together and integrates them to substantially reduce the manufacturing cost of ethylene.
There are several different types of NGL plants including mechanical refrigeration, lean oil and cryogenic turboexpander. Of these, the cryogenic turboexpander plant is the most common and the only one capable of deep ethane recovery. Within the cryogenic turboexpander family there are actually two basic types of plants, the standard types and the refluxing types. The standard type, which is representative of earlier designs, has the expander discharge entering the top of the demethanizer and has an ethane recovery of approximately 65 to 75%. The refluxing types, which are a more modern design, have the expander discharge entering the middle of the demethanizer and a second stream that has been condensed in a reflux condenser entering or (refluxing) the top of the tower. These designs have an ethane recovery of approximately 85% to 99%. In the subsequent review of the prior art, the more modern refluxing type of cryogenic turboexpander gas plant is capable of 95+% ethane recovery.
Today, there are numerous ethylene plant technologies. Each one similar but different enough such that they can claim advantage over the other. Instead of reviewing each such technology, for purposes of disclosure of the prior art, a simple design, representative of current technology has been chosen. The main ethylene plant design as well as the supporting ethylene and propylene refrigeration systems are discussed in detail in later sections. While NGL gas plants and ethylene plants have operated side by side, to applicant's knowledge, components of these two plants have never been integrated to substantially reduce the capital and operating costs of manufacturing ethylene.